MODELING & SIMULATION OF BIOMASS GASIFIER: EFFECT OF OXYGEN ENRICHMENT AND STEAM TO AIR RATIO

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1 MODELING & SIMULATION OF BIOMASS GASIFIER: EFFECT OF OXYGEN ENRICMENT AND STEAM TO AIR RATIO ABSTRACT B. V. Babu* & Pratik N. Sheth Chemical Engineering Group Birla Institute of Technology & Science, Pilani Rajasthan, India. Gasification is one of the efficient ways to convert the energy embedded in biomass. Understanding of the effect of a few key parameters such as oxygen enrichment and preheating of air on major design parameters is crucial in designing of a biomass gasifier. In the present study, equilibrium modeling is used to predict the performance of a downdraft gasifier. The composition of producer gas and, hence, the calorific values are determined. The effects of oxygen enrichment of air, preheating of air, and steam to air ratio on gas composition, reaction temperature and calorific values are investigated. The calorific values of the producer gas increase as the oxygen fraction increases and also as the steam to air ratio increases. Keywords: Biomass; Gasification; Equilibrium Modeling; Downdraft Gasifier; Simulation; Renewable Energy 1. INTRODUCTION Biomass and waste are widely recognized to be the major potential for energy production. Wood and other forms of biomass including energy crops and agricultural and forestry wastes are some of the main renewable energy resources available. Biomass fuels and residues can be converted to energy via thermal, biological and physical processes. In principle, the gasification units employed for coal can also be applied for biomass and waste, but significant differences exist between the two fuel categories. Coal pyrolysis yields 6 to 8% char while the balance coming from gases and tars. When biomass is pyrolyzed, gases and tars represent 7 to 9% of the total mass fed, whereas only 3 to 1% is a highly reactive char [1]. In the thermo-chemical conversion technologies, biomass gasification has attended the highest interest as it offers higher efficiencies compared to combustion and pyrolysis []. Gasification is the conversion of solid carbonaceous fuel into combustible gas by partial combustion. The mixture of combustible gases thus produced is called producer gas [3]. In view of the considerable interest in the gasification process worldwide, it is necessary to model and predict the performance of the gasifier in priori. Babu and Chaurasia in their studies [,5,6,7,8,9] reported extensive results on pyrolysis, which is one of the zones of a biomass gasifier. The residence time for the biomass in a gasifier is long enough. It will allow pyrolysis products to burn and subsequently to achieve an equilibrium state in the reduction zone before leaving the gasifier [1,11]. An equilibrium model has been developed and the variation with moisture content for fixed temperature was found out. An equilibrium model based on minimization of Gibbs free energy for wood waste (saw dust), has been simulated by Altafini et al. [11]. The effects of oxygen factor and moisture content of wood on gas composition, reaction temperature and calorific values are investigated. The calorific values of the producer gas decreases as the oxygen factor increases and also as the moisture content increases [1]. The effect of one of the important parameters such as oxygen enrichment of air is not reported in the literature. ence the present study focuses on developing equilibrium model and studying the effects of oxygen enrichment of air on composition, reaction temperature and calorific values of the gases. Model predictions are also compared with the experimental data reported by Jayah et al. [13]. For fixed oxygen factor the effects of preheating of air on *Corresponding author: Assistant Dean Engineering Services Division & ead - Chemical Engineering Department bvbabu@bits-pilani.ac.in omepage: Phone: Ext 5 Fax:

2 the reaction temperature is also studied. The effect of saturated steam gasification along with dry air is included in the equilibrium modeling and the variation with steam to air ratio is found out.. MODEL The equilibrium model assumes that all the reactions are in thermodynamic equilibrium. It is expected that the pyrolysis product burns and achieves equilibrium in the reduction zone before leaving gasifier; hence an equilibrium model can be used in the downdraft gasifier [1]. The reactions are as follows: CO O CO (1, J/mol) C C (75, J/mol) The equilibrium constant for methane generation (K 1 ) is K 1 = P C ( ) P (1) And equilibrium constant for shift reaction (K ) is P CO P K = P P CO O The typical chemical formula of woody material, based on a single atom of carbon, is C 1. O.66. () The global gasification reaction can be written as follows: C 1. O.66 w O mo 3.76mN = x 1 x CO x 3 CO x O x 5 C 3.76mN (3) Where w is the amount of water per kmol of wood, m is the amount of oxygen per kmol of wood, x 1 to x 5 are the coefficients of constituents of the products. For the known moisture content, the value of w becomes a constant and m can be found out from the airflow rate per kmol of wood. From the global reactions, there are six unknowns x 1 to x 5, and T, representing the five unknown species of the product and the temperature of the reaction. Therefore six equations are required, which can be obtained from the following material and energy balances. Carbon Balance: 1 = x x 3 x 5 () ydrogen Balance: w 1. = x 1 x x 5 (5) Oxygen Balance: w.66 m = x x 3 x (6) The heat balance for gasification process (assumed to be adiabatic) is: fwood w 3.76m fn ( f ( ) ( ) ) O l m fo T '( mc 3.76mC ) po pn x1 f x fco x3 fco x f ( ) ( O x5 fc T x 1C p = xc x3c x ( ) C po x5c 3.76 mc pn )] pc (7) Where T = T T 1, & T =T T 1

3 T 1 = temperature of the inlet, T = temperature of the reduction zone T = air inlet temperature If steam were also fed to the gasifier then energy balance would be modified as follows. fwood 3.76 T ' w( f ( ) ( ) ) O l m fo m fn s( ( ) '' ( ) ) fo g T C po ( mc 3.76mC ) po pn And in the material balance equations w would be replaced by w s. Where T = T T 1 s = kmol of steam per kmol of wood T the steam temperature T 1 the ambient temperature x1 f x fco x3 fco x f ( ) ( O x5 fc T x 1C p = xc x3c x ( ) C po x5c 3.76 mc pn )] pc (8) From Eq. () x 5 = 1 x x 3 (9) From Eq. (5) x = w.7 - x 1 - x 5 (1) Substituting the value of x 5 from the Eq. () into Eq. (5) x = x 1 x x 3 w 1.8 (11) From Eq. (1) x 1 K 1 = 1 x x 3 (1) Substituting the value of x from the Eq. (11) into Eq. (6) x 1 3x x 3 = m 1.9 (13) Substituting the value of x from the Eq. (11) into Eq. () x 1 x 3 = K x [ x 1 x x 3 w 1.8 ] (1) From Eq. (7), T x fwood w ( ) 3.76m T '( mc 3.76mC ) x f O( l ) ( ) 1 f fco 3 fco f O( ) 5 fc = T1 (15) [( x1c p xc x3c xc p O( ) x5c pc 3.76mC pn )] x x fo The general equation for lnk 1 [1] is given by ln K 1 = ( 6.567) lnt T T 3.51 (16) T 6 ( T ) The general equation for lnk [1] is given by ln K = 1.86lnT.7 1 T 18.7 (17) T ( T ) fn x The set of equations (1) to (17) can be solved using the following algorithm: po pn

4 1. Specify the value of m and w.. Assume temperature T, find K 1 & K using Eq. (16) and Eq. (17). 3. Find x 1, x, & x 3 using Eq. (1), Eq. (13), & Eq. (1) respectively.. Find x & x 5 using Eq. (9) & Eq. (11) respectively. 5. Calculate the new value of T using Eq. (15). 6. Repeat the above steps until successive value of T becomes constant. 3. RESULTS AND DISCUSSION Model predictions are compared with the experimental data reported by Jayah et al. [13]. Composition of the producer gas is compared and shown in the Fig.1. Experimentally reported compositions are for air flow rate of 55.6 kg/hr, wood rate of 18.6 kg/hr, and wood moisture rate of 3. kg/hr [13]. Using these values, oxygen factor and initial moisture content are found to be of.5 and 15.5% respectively. These values are used to predict the gas compositions and compared with the experimental data. Fig.1 shows that compositions of all components are in good agreement with experimentally reported data. A sensitivity analysis of the model results is carried out, by varying the oxygen content of air, preheated temperature of air, and the steam to air ratio..7 Gas compositions Equilbrium model predictions Expt results comp_ comp_co comp_co comp_c comp_n Fig.1 Model comparison with expt. data of Jayah et al. 3.1 Oxygen Enrichment of Air C x O y mo CO.5 x O with m = 1.5 x.5 y Oxygen factor (F) is the O fraction of stoichiometric O amount used in a neutral and theoretical combustion process. For wood the values are x = 1. and y =.66, which gives m = 1.3 (stoichiometric value). The gasification process takes place when there is a lack of O, let us take an O amount equal to the ¼ of the stoichiometric in a theoretical amount in a theoretical combustion, that is F = 5.75% [1]. Along with m moles of O, (.79/.1)m moles of N would be entering in the gasifier. For air with an oxygen content of 3% by volume, along with m moles of O, (.7/.3)m moles of N would be entering into the gasifier. There would be a decrease of N moles entering the gasifier for oxygen-enriched air. 3. The influence of the Oxygen Enrichment Fig. shows how the composition of gas changes with oxygen fraction in the air for an oxygen factor of.3 and initial moisture content of 1% with no preheating of air. Mostly all variations of the molar fractions versus

5 oxygen fractions are more or less linear. The mole fraction of N decreases with increasing oxygen fraction as expected. The composition of methane produced is very low. The percentage of hydrogen in the fuel gas increases continuously with oxygen fraction from about % to 8% for an increase of oxygen fraction from 5% to 5%. A similar trend is also observed for carbon monoxide. It is interesting to know that carbon dioxide and water or percentages are also increasing as nitrogen percentage are decreasing. In producer gas, nitrogen, which is an inert, reduces and other component fractions would increase as is evident from Fig.. Composition Oxygen Fraction (Vol) N C O CO CO Fig. Effect of oxygen enrichment on the composition Fig. 3 shows that the reaction temperature goes up from 19 K to 1 K when oxygen fraction increases from 5% up to 5%. This is due to the increased oxygen and thereby decreased amount of N, which generally acts as a heat carrier. Temperature (K) Oxygen Fraction ( vol.) Fig. 3 Effect of oxygen enrichment on reaction temperature Fig. shows a significant increase in the calorific values of fuel gas by increasing the oxygen fraction. Calorific value increases nonlinearly from 1665 kj/m 3 to 1 kj/m 3 for an increment of oxygen fraction.5 to.5. Calorific value increment is due to increase in the amount of CO and of. Amount of oxygen required to enhance particular amount of calorific value is calculated. To increase the calorific value by 55 kj/m 3, oxygen fraction of.35 is needed, which can be inferred from Fig.. Based on the cost estimation, it was found that the added extra energy per unit cost of investment for oxygen to enrich air is 35 kj/rs.

6 Calorific Value (kj/m3) The Effect of preheating of air Oxygen Fraction ( Vol) Fig. Effect of Oxygen Enrichment on Calorific Values Fig. 5 shows the effect of preheating temperature on reaction temperature. Reaction temperature increases from 19 K to 155 K for preheating from ambient temperature to 8 K. The variation is linear and calorific values and gas composition changes very slightly with preheating of air. Preheating of air is useful to increase reaction temperature and may be employed in biomass gasifier when reaction temperature falls down due to high moisture content of biomass. 13 Reaction Temperature (K) Preheat Temperature of Air ( K) Fig. 5 Effect of Preheating of air on reaction temperature 3. Steam to air ratio variation and its effect on the composition In some gasifiers, the injection of steam in the bed allows controlling reaction temperature and favors the ydrogen production by water gas shift reaction. At the same time carbon monoxide amount decreases.

7 Composition kg water / kg dry air Fig. 6 Influence of steam to air ratio on gas compositions CO CO O C N 88 Calorific Value (kj/m3) kg water/ kg dry air Fig. 7 Effect of steam on calorific value Fig. 6 shows the change of composition as steam to air ratio increases for oxygen factor of.5 and without oxygen enrichment and preheating of air. Fig. 6 clearly indicates the increment of hydrogen from 9% to 13% for a steam to air ratio of to.1. It also shows a decrement of CO from 17 % to 8% for the same change of steam to air ratio. Due to this calorific values decreases and it is shown in Fig. 7. Methane increases very little for a steam to air ratio decrement. Fig. 8 indicates the nonlinear temperature variation with steam to air ratio. Temperature decreases from 16 K to 1 K for a steam supply of to.1 kg water/ kg dry air. Steam gasification can be used to decrease the reaction temperature.

8 Temp (K) kg water/ kg dry air Fig. 8 Effect of steam on the reaction temperature. CONCLUSIONS The modeling of gasification process in a downdraft gasifier is performed using an equilibrium model. The calculations of the composition and the calorific value of the producer gas with wood as a raw material are illustrated. From the sensitivity analysis for the oxygen enrichment in the air, preheating of air, and the steam to air ratio, following conclusions are drawn: 1. The content of hydrogen in producer gas increases with oxygen fraction and also with increment in steam to air ratio.. The carbon monoxide content in producer gas increases with oxygen fraction and decreases nonlinearly with steam to air ratio. 3. The methane content in producer gas increases with steam to air ratio and also with oxygen fraction. The amount of methane is insignificantly low in value to increase the calorific value.. The reaction temperature increases with oxygen fraction and decreases with steam to air ratio, almost in a linear fashion. The reaction temperature also increases for preheated air intake. 5. The calorific value increases with increasing oxygen fraction and decreases with steam to air ratio. These conclusions suggest that by using air with enriched oxygen gives higher calorific values of producer gas. There is very less effect on gas composition of preheating the air. Steam gasification may be desirable for production but not useful for producer gas generation as it degrades it. The results of this study are very useful in choosing the appropriate controlling parameters, while operating a downdraft biomass gasifier. Nomenclature C p,i Specific heat of component i (kj/mol) F Oxygen factor eat of formation of component i (kj/mol) K m P i T w s f,i References Equilibrium constant Moles of oxygen per mole of wood Partial pressure of component i (kpa) Temperature (K) Moles of water per mole of wood Moles of steam per mole of wood [1] C. D. Blasi, Dynamic Behavior of stratified downdraft gasifiers, Chemical Engineering Science, Vol. 55, pp.931-9,. [] A.V. Bridgwater, th International Symposium on Waste Treatment Technologies (Thermal, Non- Thermal and Clean-up), Sheffield, UK. 9 June - July 3

9 [3] G. Bhave, Technical notes in the Proceedings of Intensive workshop on - Testing Evaluation of Biomass Gasifier Systems and Related Laboratory Investigation, (SPRERI), Vallabh Vidyanagar, (Apr, 1). [] B.V. Babu, A.S. Chaurasia, Modeling, simulation, and Estimation of Optimum Parameters in Pyrolysis of Biomass, Energy Conversion and Management, Vol., pp , 3 [5] B.V. Babu, A.S. Chaurasia, Modeling for Pyrolysis of Solid Particle: Kinetics and eat Transfer Effects" Energy Conversion and Management, Vol., pp , 3. [6] B.V. Babu, A.S. Chaurasia, Parametric Study of Thermal and Thermodynamic Properties on Pyrolysis of Biomass in Thermally Thick Regime, Energy Conversion and Management, Vol. 5, pp. 53-7,. [7] B.V. Babu, A.S. Chaurasia, Pyrolysis of Biomass: Improved Models for Simultaneous Kinetics & Transport of eat, Mass, and Momentum, Energy Conversion and Management, Vol. 5, pp ,. [8] B.V. Babu, A.S. Chaurasia, Dominant Design Variables in Pyrolysis of Biomass Particles of Different Geometries in Thermally Thick Regime, Chemical Engineering Science, Vol. 59, pp ,. [9] B.V. Babu, A.S. Chaurasia, eat Transfer and Kinetics in the Pyrolysis of Shrinking Biomass Particle, Chemical Engineering Science, Vol. 59, pp ,. [1] Z. A. Zainal, R. Ali, C.. Lean, K. N. Seetharamu, Prediction of performance of a downdraft gasifier using equilibrium modeling for different biomass materials, Energy Conversion and Management, (1) M. Philippe, R. Dubuisson, Energy Conversion and Management, Vol. 3, pp ,. [11] R. Altafini, P. Wnader, R. M. Barreto, Prediction of the working parameters of a wood waste gasifier through an equilibrium model, Energy and Conversion Management, Vol., pp , 3. [1] B.V. Babu, P.N. Sheth, Modeling and Simulation of downdraft biomass gasifier, presented at the 57 th Indian chemical engineering congress, chemcon-. [13] T.. Jayah, Lu Aye, R.J. Fuller, D.F. Stewart, Computer simulation of a downdraft wood gasifier for tea drying, Biomass and Bioenergy, Vol. 5, pp.59-69, 3. [1] M. Philippe, R. Dubuisson, Performance analysis of a biomass gasifier, Energy Conversion and Management, Vol.3, pp ,.